Biomechanical characterization of the three-dimensional kinematic behaviour of the Dynesys dynamic stabilization system: an in vitro study

Abstract

The Dynesys, a flexible posterior stabilization system that provides an alternative to fusion, is designed to preserve intersegmental kinematics and alleviate loading at the facet joints. Recent biomechanical evidence suggests that the overall range of motion (ROM) with the Dynesys is less than the intact spine. The purpose of this investigation was to conduct a comprehensive characterization of the three-dimensional kinematic behaviour of the Dynesys and determine if the length of the Dynesys polymer spacer contributes to differences in the kinematic behaviour at the implanted level. Ten cadaveric lumbar spine segments (L2-L5) were tested by applying a pure moment of +/-7.5 Nm in flexion-extension, lateral bending, and axial rotation, with and without a follower preload of 600 N. Test conditions included: (a) intact; (b) injury; (c) injury stabilized with Dynesys at L3-L4 (standard spacer); (d) long spacer (+2 mm); and (e) short spacer (-2 mm). Intervertebral rotations were measured using an optoelectronic camera system. The intersegmental range of motion (ROM), neutral zone (NZ), and three-dimensional helical axis of motion (HAM) were calculated. Statistical significance of changes in ROM, NZ, and HAM was determined using repeated measures analysis of variance (ANOVA) and Student-Newman-Keuls post-hoc analysis with P<0.05. Implantation of the standard length Dynesys significantly reduced ROM compared to the intact and injured specimens, with the least significant changes seen in axial rotation. Injury typically increased the NZ, but implantation of the Dynesys restored the NZ to a magnitude less that that of the intact spine. The Dynesys produced a significant posterior shift in the HAM in flexion-extension and axial rotation. The spacer length had a significant effect on ROM with the long spacer resulting in the largest ROM in all loading directions without a follower preload. The largest differences were in axial rotation. A 4 mm increase in spacer length led to an average intersegmental motion increase of 30% in axial rotation, 23% in extension, 14% in flexion, and 11% in lateral bending. There were no significant changes in NZ with different spacer lengths. Typically, the short spacer caused a greater shift and a greater change in orientation of the HAM than the long spacer. The long spacer resulted in a ROM and a motion pattern, as represented by the HAM, that was closer to that seen in an intact specimen. The results of this study suggest that the length of the Dynesys spacer altered the segmental position and therefore affected kinematic behaviour.

Fig. 1

Dynesys polycarbonate urethane (PCU) spacer surrounding tensioned polyethylene terephthalate (PET) cord between titanium alloy pedicle screws

Fig. 2

Test set-up. Looking at the posterior aspect of the specimen with the implanted Dynesys (see reference [7] for details)

Fig. 3

Local coordinate system created for L3 and L4 for HAM analysis. Origin is at the antero-superior point of L4

Fig. 4

Helical axis of motion (HAM) in flexion–extension with no follower load. The results presented are the average for ten specimens and have been normalized by the height, width, and length of the L4 vertebral body

Fig. 5

Helical axis of motion (HAM) in axial rotation with no follower load. The results presented are the average for ten specimens and have been normalized by the height, width, and length of the L4 vertebral body

Fig. 6

Helical axis of motion (HAM) in lateral bending with no follower load. The results presented are the average for ten specimens and have been normalized by the height, width, and length of the L4 vertebral body

Fig. 7

Average rotations with the three different Dynesys spacers (short, standard, and long), normalized to intact ROM. All the three directions of loading, with and without a follower preload

Fig. 8

Average neutral zone for the three different Dynesys spacers (short, standard, and long), normalized to intact neutral zone. All three directions of loading, with and without a follower preload